Currently microwave ovens are often used for defrosting and reheating foodstuffs which have been previously kept in a freezer. In general, this defrosting is effected empirically i.e. the user determines the approximate weight of the food to be defrosted in order to derive an approximate operating time for the microwave oven. This results in more or less complete defrosting or even a beginning of cooking.
It is also known from the literature that around 2.45 GHz, the microwave absorption of water, which is the principal constituent of most foodstuffs, differs considerably depending on whether the water temperature is below or above 0.degree. C. The ice below 0.degree. C. is highly transparent to microwaves and the water at a temperature above 0.degree. C. has a very strong microwave absorption. This effect is caused by variations of dielectric losses of water as a function of temperature. French Patent 2,571,830 describes a microwave oven provided with a standard load placed in the oven beside the food to be processed. The standard load absorbs microwave energy in accordance with a distribution which depends on the standard load and the load of food to be processed.
Thus, from the rise in temperature of the standard load it is possible to derive the quantity of food present in the oven and to automatically determine the cooking time. According to said patent the rate of heating of the standard load is substantially independent of the temperature of the detector.
Although a defrosting operation is mentioned therein said patent does not reveal any means for detecting the critical transition from a frozen condition to a defrosted condition of the food to be processed, or how the defrosting can be detected and controlled.